Re: [RFC] nvme-mpath: delete disk after last connection

[Hannes Reinecke <hare@suse.de>]

On 9/25/20 11:38 PM, Keith Busch wrote:
> I have this tagged "RFC" because I'm not sure if there's a reason why the code
> is done the way that it is today.
> 
> The multipath code currently deletes the disk only after all references
> to it are dropped rather than when the last path to that disk is lost.
> This has been reported to cause problems with some usage, like MD RAID.
> 
> Delete the disk when the last path is gone. This is the same behavior we
> currently have with non-multipathed nvme devices.
> 
> The following is just a simple example that demonstrates what is currently
> observed using a simple nvme loop back (loop setup file not shown):
> 
>   # nvmetcli restore loop.json
>   [   31.156452] nvmet: adding nsid 1 to subsystem testnqn1
>   [   31.159140] nvmet: adding nsid 1 to subsystem testnqn2
> 
>   # nvme connect -t loop -n testnqn1 -q hostnqn
>   [   36.866302] nvmet: creating controller 1 for subsystem testnqn1 for NQN hostnqn.
>   [   36.872926] nvme nvme3: new ctrl: "testnqn1"
> 
>   # nvme connect -t loop -n testnqn1 -q hostnqn
>   [   38.227186] nvmet: creating controller 2 for subsystem testnqn1 for NQN hostnqn.
>   [   38.234450] nvme nvme4: new ctrl: "testnqn1"
> 
>   # nvme connect -t loop -n testnqn2 -q hostnqn
>   [   43.902761] nvmet: creating controller 3 for subsystem testnqn2 for NQN hostnqn.
>   [   43.907401] nvme nvme5: new ctrl: "testnqn2"
> 
>   # nvme connect -t loop -n testnqn2 -q hostnqn
>   [   44.627689] nvmet: creating controller 4 for subsystem testnqn2 for NQN hostnqn.
>   [   44.641773] nvme nvme6: new ctrl: "testnqn2"
> 
>   # mdadm --create /dev/md0 --level=mirror --raid-devices=2 /dev/nvme3n1 /dev/nvme5n1
>   [   53.497038] md/raid1:md0: active with 2 out of 2 mirrors
>   [   53.501717] md0: detected capacity change from 0 to 66060288
> 
>   # cat /proc/mdstat
>   Personalities : [raid1]
>   md0 : active raid1 nvme5n1[1] nvme3n1[0]
>         64512 blocks super 1.2 [2/2] [UU]
> 
> Now delete all paths to one of the namespaces:
> 
>   # echo 1 > /sys/class/nvme/nvme3/delete_controller
>   # echo 1 > /sys/class/nvme/nvme4/delete_controller
> 
> We have no path, but mdstat says:
> 
>   # cat /proc/mdstat
>   Personalities : [raid1]
>   md0 : active (auto-read-only) raid1 nvme5n1[1]
>         64512 blocks super 1.2 [2/1] [_U]
> 
> And this is reported to cause a problem.
> 
> With the proposed patch, the following messages appear:
> 
>   [  227.516807] md/raid1:md0: Disk failure on nvme3n1, disabling device.
>   [  227.516807] md/raid1:md0: Operation continuing on 1 devices.
> 
> And mdstat shows only the viable members:
> 
>   # cat /proc/mdstat
>   Personalities : [raid1]
>   md0 : active (auto-read-only) raid1 nvme5n1[1]
>         64512 blocks super 1.2 [2/1] [_U]
> 
> Reported-by: Hannes Reinecke <hare@suse.de>
> Signed-off-by: Keith Busch <kbusch@kernel.org>
> ---
>   drivers/nvme/host/core.c      | 3 ++-
>   drivers/nvme/host/multipath.c | 1 -
>   drivers/nvme/host/nvme.h      | 2 +-
>   3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 4857168f71f2..a2faa3625e39 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -475,7 +475,8 @@ static void nvme_free_ns_head(struct kref *ref)
>   	struct nvme_ns_head *head =
>   		container_of(ref, struct nvme_ns_head, ref);
>   
> -	nvme_mpath_remove_disk(head);
> +	if (head->disk)
> +		put_disk(head->disk);
>   	ida_simple_remove(&head->subsys->ns_ida, head->instance);
>   	cleanup_srcu_struct(&head->srcu);
>   	nvme_put_subsystem(head->subsys);
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 74896be40c17..55045291b4de 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -697,7 +697,6 @@ void nvme_mpath_remove_disk(struct nvme_ns_head *head)
>   		 */
>   		head->disk->queue = NULL;
>   	}
> -	put_disk(head->disk);
>   }
>   
>   int nvme_mpath_init(struct nvme_ctrl *ctrl, struct nvme_id_ctrl *id)
> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index a42b75869213..745cda1a63fd 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -670,7 +670,7 @@ static inline void nvme_mpath_check_last_path(struct nvme_ns *ns)
>   	struct nvme_ns_head *head = ns->head;
>   
>   	if (head->disk && list_empty(&head->list))
> -		kblockd_schedule_work(&head->requeue_work);
> +		nvme_mpath_remove_disk(head);
>   }
>   
>   static inline void nvme_trace_bio_complete(struct request *req,
> 
I'm okay with that in general, but then again we might run into 
situations where an 'all paths down' scenario is actually expected 
(think of a temporary network outage on nvme-tcp).
So I guess we need to introduce an additional setting 
(queue_if_no_path?) to be specified during the initial connection.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

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